Hello,
I have an issue with axis aligning…
So, i want this axis (Global):
I want that axis to be perfectly 90°, in order to move that edge inwardly.
Thanks!
If you just want to be able to move the edge that way and not change its normal direction , use the below method.
Select the face below it and create a new " transform orientation " . See image.
Once you have that, you can just select that from the drop down when you need it.
There will be simpler way just to manipulate once or extrude it (use the top ortho view for example).
I’m using 2.79 but i tried it in 2.82 and it does nothing, i mean it has the same orientation as Normal Orientation. 
I’ve done a quick rough replica of your object, and removed the bottom face.
Do you want to rotate/move just that one edge, or the face?
Or all of them?
Is this what you’re trying to achieve??..
After removing the bottom face, in Edit Mode > Edge Select.
Select all bottom edges and activate the Scale Tool.
Either use the gizmo or type in the X,Y, scale proportions in the Resize popup, 0.9 for both X and Y appears to create that 90° angle.
(This technique will still work with the bottom face intact)
Hope this helps somehow 
This shows the bottom face intact with Face Normals at 90°
I’m not sure I understood it correctly, but you can press G twice (GG) to slide or use shift + numpad 7,1,3 to align the selection with the view.
No, this is not it.
What i’m trying to do is this part of the exhaust from this ship: Spaceship (it’s on sketchfab.com)
The big trapezoid cylinder part:
I found a way to do it with boolean but it’s tedious and it would be ridiculous to not be able to do this in Blender (orientating the axis as you will).
Like in this video: Cinema 4d - Change Axis Point Of An Object
Really simple and very usefull.
Sorry that i was not more specific in the first place.
Edit: Fellow member floatharr pointed out that this might not be what you want, but nevertheless, I will leave this video up because others might find it useful!
Okay, what the c4d tutorial shows you will get if you check the origins. All transformations work on the origins then. So don’t forget to uncheck the box. This is a new feature that was requested by people from other 3D apps. Blenderer usually use the 3D cursor for this 
There’s a lot of good answers here already, but I believe you’re approaching this the wrong way. If it seems like something is more difficult than it should be there’s usually a much better way of doing it. Here’s my attempt:
Here’s a gif of the process: 
Regarding the problem i encountered, yes, this is the solution.
It took me a while to check it since i`m not using Blender 2.8x yet (it seems i have to switch asap).
But imagine doing this for several edges/faces.
It’s kinda frustrating that there isn’t a quicker way (i think you agree with me on this one).
On the other hand, yes, there are other ways like the one @Floatharr pointed out, i just hope there are other ways when it comes to a problem like this one (which is not easy to resolve).
Thank you all for the answers!
I use angle tool for such things as @Floatharr describes. It is more intuitive but his method is faster and takes many steps at once.
Oh, ok. I`ll look into that as well, thanks!
